1. Field of the Invention
The present invention relates to a method for producing silicon that is suitable for use in making solar cells. This technology enables effective production of highly purified silicon for use in solar cells.
2. Description of the Related Art
Elemental impurities such as P, B, C, Fe, Al, Ti and O in silicon for use in solar cells need to be controlled. Impurities of P, B, Fe, Al and Ti should be below about 0.1 ppm, and impurities of C and O should be about 5 to 10 ppm to ensure desired energy conversion efficiency. In addition, large amounts of purified silicon should be available inexpensively in order to make wide use of it in solar cells.
Conventionally, silicon for use in solar cells has been mainly produced by about the same methods as silicon intended for use in semiconductors; i.e., by gas-phase methods as shown, for example, in FIG. 3 of the appended drawings. The method comprises reducing highly purified silicon oxide (SiO.sub.2) with high purity carbon to produce a liquid of so-called crude metallurgical grade silicon, which has low purity; converting the metallurgical grade silicon into a silane compound; highly purifying the silane compound by distillation; and forming a substrate by solidification after purifying silicon by precipitation. This gas-phase method for mass-production not only has a high production cost, but its yield is low. It has such a high purity that elemental impurities such as B must even be added later.
In another conventional method for purifying silicon that is excellent for use in solar cells, using metallurgical grade silicon as a starting material, purification may be accomplished by use of a following metallurgical process.
As shown for example in FIG. 4A of the drawings, some of the metallic elemental impurities (such as Al, Ti, Fe and the like) are removed by directional solidification after first eliminating P by directional vacuum refining, then finish refining by again directionally melting the silicon again under refining conditions to remove B and C by oxidative refining, and applying by finish solidification purification which serves both to eliminate metallic impurities after de-oxidation and to produce an ingot. In other words, metallic impurities such as Al, Fe, Ti and the like in the metallurgical grade silicon are eliminated by two directional solidifications, taking advantage of the small solid-liquid partition coefficient of the impurities. C is precipitated on the surface during the solidification step or is eliminated as CO gas when C is present as a solid solution; P is eliminated in vacuum by taking advantage of its high vapor pressure; and B is eliminated by oxidative purification by adding H.sub.2 O, CO.sub.2 or O.sub.2. This metallurgical process enables mass-production with large scale facilities.
However, important problems remain. A different refining treatment is required for removing each impurity. Two complicated solidification purification steps are needed, and the process has low yield due to twice cutting off the top portion of the silicon ingot. There is also the high cost of electricity.
In each solidification step of FIG. 4A, metallic impurities tend to concentrate at one location in the molten metallurgical grade silicon as the solidification step nears its end. After each solidification the portion of the ingot that contains the concentrated portion of the impurities is cut off and discarded.
Since the cut-off portions account for about 20% of the solidified ingots, removal of these portions lowers the silicon yield. Productivity of the process shown in FIG. 4A would be greatly improved, and silicon for use in solar cells could be produced at a lower cost, if these silicon portion could be recycled.